Differential input probes are commonly used to acquire a nominally differential signal, such as on a high-speed serial data bus, from a device under test (DUT) using a single test and measurement instrument channel. TriMode™ input probes have the additional capability of acquiring the common-mode signal, or either side of the differential pair signal as a single-ended signal.
An ideal differential signal includes two complementary signals sent on two separate wires. Any skew, or difference in delay, between the two sides of wiring in a DUT to the test and measurement instrument and/or within the test and measurement instrument itself causes mode conversion of the differential signal. Mode conversion is when a portion of the differential signal appears as the common-mode signal, or vice versa. Mode conversion due to skew grows progressively worse at higher frequencies. For instance, just one picosecond (ps) of skew at 25 GHz will lead to over 15% of the common-mode voltage appearing as a differential signal.
One approach to minimizing errors due to the skew-related mode conversion is to provide mechanically variable delays, e.g., trombones, in one or both probe inputs, and mechanically adjusting the mechanically variable delays to match in delay. In some cases, the adjustment may be done at probe manufacture to compensate for the mismatch in the probe cable. However, this does not help with the skew in the user's wiring from the DUT to the probe. Alternatively, the adjustment may be user-accessible, but would still be inconvenient for a user to adjust. Trombones also add to the cost of the probe and may lead to reliability issues.
Another approach to minimize the errors due to skew-related mode conversion is to use two test and measurement instrument channels to separately acquire each side of the differential pair signal. Digital signal processing is used to adjust the timing of a first channel's waveform to match the timing of a second channel's waveform. Then, the two waveforms are subtracted to find the differential signal. This method provides a form of tri-mode functionality, in that the common-mode or either single-ended signal is available depending on the math function used. However, this method is quite costly due to the need to use two channels in place of one.
Embodiments of the disclosed technology address these and other limitations in the prior art.